Printing presses, which are known in the art, print a design on a substrate. Typically, the design is composed of several color designs, each associated with a respective color. Each color design exhibits a respective topography. The printing press prints each color design in a predetermined order, thus producing the design on the substrate (i.e., the printed design). Typically, a design to be printed is provided in the form of a file (e.g., in Portable Document Format—PDF), which includes information relating to the different color designs (i.e., also referred to as ‘layers’) from which the design is composed, as well as a composite image of the design (e.g., an RGB image). In essence, each layer is a gray level image of the respective color design. In other words, each layer includes the topography associated with the respective color design thereof.
Inspection of designs, printed on a substrate by a printing press, is also known in the art and aims at detecting defects in the printed design. Defects in cylinder printing presses (e.g., rotogravure, flexographic printing press or an offset printing press) may take the form of hickies, voids, pinholes, slurring, mottles and the like. In digital printing presses, defects may be caused by a missing nozzle (i.e., a nozzle that did not print), a deviating nozzle (i.e., a nozzle that printed at a location where it was not intended to print), a redundant nozzle (i.e., a nozzle that printed when it was not intended to print) or an inconsistent nozzle, also referred to as “weak nozzle” (i.e., a nozzle which does not deposit ink at a predetermined rate). The defects may take the form of streaks in the printed image. Inspection is typically performed by acquiring an image of the printed design and comparing this acquired image to a reference image also known as a ‘golden master’ or ‘proof’. Such reference images are usually generated by printing a copy of the composite design after the printing press has been set up and the composite design is judged by a person to be of sufficient quality, and then acquiring an image of the printed design with an imager. The reference image is typically a Red Green and Blue (RGB) image. It is noted that the number of layers does not necessarily correspond to the number of dimensions of the color space employed by the imager (e.g., three in the case of an RGB imager).